1. Field of the Invention
The present invention relates to an engine, and more specifically to an engine having a variable valve mechanism arranged to switch a lift degree of a valve between a high speed state and a low speed state.
2. Description of the Background Art
JP 2002-303109 discloses a speed range selectable valve mechanism for an internal combustion engine. In this valve mechanism, a camshaft includes low and high cam noses, and a valve supported at a cylinder head is selectively engaged with one of the high and low cam noses by the cam-linkage, which allows the valve to be opened or closed according to the high and low speed ranges of the internal combustion engine. First and second rocker arms are pivotally supported at the cylinder head. The swinging end of the first rocker arm and the low cam nose are engaged with each other by the cam-linkage, and the swinging end of the second rocker arm and the high cam nose are engaged with each other by the cam-linkage. A columnar engaging member (connecting pin) is supported at the first rocker arm so that it can slide therein in a reciprocating manner. The engaging member slides and projects from the side of the first rocker arm to the side of the second rocker arm in such a manner that it can advance/withdraw. This allows the first and second rocker arms to be detachably engaged with each other. The cylinder head is provided with a hydraulic actuator that applies an external force upon the engaging member against the energizing force of an engagement releasing spring. The actuator includes a cylinder hole (hydraulic cylinder) formed at the cylinder head and a piston (hydraulic piston) snugly inserted into the cylinder hole so that the piston can slide in the hole in a reciprocating manner. The cylinder hole leads to the hydraulic pump through an oil passage. The oil passage is formed at the cylinder head. A coil-shaped rocker arm spring (lost motion spring) is fitted onto the pivotal shaft of the rocker arm and the spring urges the second rocker arm so that the second rocker arm and the high cam nose are engaged with each other by the cam-linkage.
The hydraulic actuator is provided between the valve springs in the valve mechanism but the spacing between the valve springs is small in a small size engine and therefore there is little free space. Since the cylinder hole (hydraulic cylinder) is formed at the cylinder head, it is difficult to form the cylinder holes with high precision in a multi-cylinder engine. It is also difficult to assemble the piston of the hydraulic actuator and the rocker arm. In addition, the complicated oil passage is difficult to form.
JP 10-18826 A discloses a variable mechanism capable of carrying out various kinds of switching about the opening/closing timing, the lift degree, and the stopping timing for intake or exhaust valves in an internal combustion engine. In the variable valve mechanism, five supports are attached in such locations that they hold the four cylinders among them, and a rocker shaft is inserted through these supports. One T-shaped low speed rocker arm is swingably provided at the rocker shaft for each cylinder. A camshaft is rotatably supported at each support, and a low speed cam used to swing the low speed rocker arm is provided at the camshaft. The variable valve mechanism includes a switching device used to switch the opening/closing timing and lift degree of a valve between two stages, i.e., the high speed state and the low speed state. The switching device includes a high speed rocker arm that is adjacent to the low speed rocker arm, swingably provided at the rocker shaft and does not directly push the valve, a high speed cam that swings the high speed rocker arm, and a hydraulic piston driving a switch pin that connects or disconnects the high speed rocker arm and the low speed rocker arm between each other.
However, the low speed rocker arm in the valve mechanism pushes two valves together in the same cylinder and therefore different lift degrees cannot be set for these valves. In addition, the low speed rocker arm is provided at the bore center and therefore the camshaft cannot be supported at the bore center. Therefore, the supporting rigidity of the camshaft is low and the valve mechanism is not suitable for high speed engines.